Method of producing silicon wafer and silicon wafer

ABSTRACT

The present invention provides a method for producing a silicon wafer, which comprises growing a silicon single crystal ingot having a resistivity of 100 Ω·cm or more and an initial interstitial oxygen concentration of 10 to 25 ppma and doped with nitrogen by the Czochralski method, processing the silicon single crystal ingot into a wafer, and subjecting the wafer to a heat treatment so that a residual interstitial oxygen concentration in the wafer should become 8 ppma or less, and a method for producing a silicon wafer, which comprises growing a silicon single crystal ingot having a resistivity of 100 Ω·cm or more and an initial interstitial oxygen concentration of 8 ppma or less and doped with nitrogen by the Czochralski method, processing the silicon single crystal ingot into a wafer, and subjecting the wafer to a heat treatment to form an oxide precipitate layer in a bulk portion of the wafer, as well as silicon wafers produced by these production methods. Thus, there is provided a DZ-IG silicon wafer in which a DZ layer of high quality is formed, and which can maintain high resistivity even if the wafer is subjected to a heat treatment for device production.

TECHNICAL FIELD

The present invention relates to a technique by which a DZ-IG siliconwafer having high resistivity and also having high gettering ability cansurely be obtained.

BACKGROUND ART

Silicon wafers of high resistivity produced by the floating zone method(FZ method) have conventionally been used for power devices such ashigh-voltage power devices and thyristors. However, it is difficult toproduce a silicon wafer having a large diameter of 200 mm or more by theFZ method, and the radial resistivity distribution of usual FZ wafers isinferior to that of CZ wafers. Therefore, silicon wafers produced by theCZ method will be promising in the future, because wafers of excellentradial resistivity distribution can be produced by the CZ method, and inaddition, wafers of a large size having a diameter of 200 mm or more cansufficiently be produced by the method.

In recent years, in particular, reduction of parasitic capacity isrequired in semiconductor devices for mobile communications and thelatest C-MOS devices. For this reason, a silicon wafer of highresistivity and a large diameter comes to be needed. Moreover, theeffect of using a high resistivity substrate on reduction oftransmission loss of signals and parasitic capacity in Schottky barrierdiodes has been reported. Furthermore, although the so-called SOI(Silicon On Insulator) wafer may be used in order to obtain furtherhigher performance of the aforementioned semiconductor devices, it isrequired to use a wafer of high resistivity produced by the CZ method asa base wafer even when semiconductor devices are produced by using theSOI wafer in order to obtain a larger diameter of wafer or solve theproblem of transmission loss of signals or the like.

However, since the CZ method utilizes a crucible made of quartz, not asmall amount of oxygen (interstitial oxygen) is introduced into asilicon crystal. Although each of such oxygen atoms is usuallyelectrically neutral, if they are subjected to a heat treatment at a lowtemperature of around 350 to 500° C., a plurality of them gather torelease electrons and become electrically active oxygen donors.Therefore, if a wafer obtained by the CZ method is subsequentlysubjected to a heat treatment at about 350 to 500° C. in the deviceproduction process and so forth, there arises a problem that resistivityof a high resistivity CZ wafer is reduced due to the formation of theoxygen donors.

In order to prevent the resistivity reduction due to the above oxygendonors and obtain a silicon wafer of high resistivity, methods forproducing a silicon single crystal having a low interstitial oxygenconcentration from an initial stage of the crystal growth by themagnetic field-applied CZ method (MCZ method) were proposed (refer toJapanese Patent Publication (Kokoku) No. 8-10695 and Japanese PatentLaid-open Publication (Kokai) No. 5-58788). Further, there has also beenproposed a method conversely utilizing the phenomenon of the oxygendonor formation, wherein a P-type silicon wafer of a low impurityconcentration and low oxygen concentration is subjected to a heattreatment at 400 to 500° C. to generate oxygen donors, and P-typeimpurities in the P-type silicon wafer is compensated by these oxygendonors so that the wafer should be converted into N-type to produce anN-type silicon wafer of high resistivity (refer to Japanese PatentPublication No. 8-10695).

However, a silicon single crystal of a low interstitial oxygenconcentration produced by the MCZ method or the like as mentioned abovesuffers from a drawback that the density of bulk defects generated by aheat treatment in the device production process becomes low, and thussufficient gettering effect will be unlikely to be obtained. In devicesof a high integration degree, it is essential to impart gettering effectby a certain amount of oxygen precipitation.

Further, the method of obtaining a silicon wafer of N-type by generatingoxygen donors by a heat treatment and compensating P-type impurities inthe wafer to convert it into N-type is a complicated method thatrequires a heat treatment for a long period of time. Moreover, thismethod cannot provide a P-type silicon wafers. In addition, this methodalso has a drawback that resistivity of wafers obtained by this methodmay vary depending on a subsequent heat treatment. Furthermore, in thismethod, in case of high interstitial oxygen concentration, it becomesdifficult to control the wafer resistivity. Therefore, this methodsuffers from a drawback that a low initial concentration of interstitialoxygen in a silicon wafer must be used, and thus the gettering effect ofthe wafer becomes low.

In order to solve these problems, the applicants of the presentapplication proposed, in a previous application (Japanese PatentApplication No. 11-241370, PCT/JP00/01124), a method for producing asilicon wafer, which comprises growing a silicon single crystal ingothaving a resistivity of 100 Ω·cm or more and an initial interstitialoxygen concentration of 10 to 25 ppma (JEIDA: Japan Electronic IndustryDevelopment Association) by the Czochralski method, processing thesilicon single crystal ingot into a wafer, and subjecting the wafer toan oxygen precipitation heat treatment so that a residual interstitialoxygen concentration in the wafer should become 8 ppma or less.According to this method, a CZ wafer of high resistivity of whichresistivity is unlikely to decrease even when the wafer is subjected toa heat treatment for device production. Therefore, if this wafer is usedas, for example, a base wafer of SOI wafer, devices of extremely highperformance for mobile communications can be obtained.

On the other hand, it is considered that, in order to realize a waferhaving performance of the same level as the SOI wafer by using a bulkwafer, of which production cost is more inexpensive compared with SOIwafer, so to speak “high resistivity DZ-IG wafer” of a structure havinga DZ layer (Denuded Zone layer) sufficiently made defect free on asurface of such a high resistivity CZ wafer is required. Although therehas conventionally been the so-called DZ-IG wafer, which is obtained bysubjecting a CZ silicon wafer having usual resistivity to a DZ-IG(Intrinsic Gettering) treatment, there has not been conceived to applythis technique to a high resistivity CZ wafer at all. Therefore, theapplicant of the present application also disclosed a method forobtaining a high resistivity DZ-IG wafer by a heat treatment that makesthe aforementioned interstitial oxygen concentration 8 ppma or less inthe previous application (Japanese Patent Application No. 11-241370).

As the DZ-IG treatment applied to a wafer of usual resistivity, athree-step heat treatment is generally used. Supersaturated oxygens inthe vicinity of a wafer surface are out-diffused by a first step hightemperature heat treatment at 1100° C. or higher, a low temperature heattreatment at around 650° C. is performed as a second step heat treatmentto form oxygen precipitation nuclei, and a moderate temperature heattreatment is performed at about 1000° C. as the third step heattreatment to allow growth of the oxide precipitates. By such athree-step heat treatment, an oxide precipitate region is formed in thewafer, and thus a DZ layer in which oxide precipitates do not exist isformed in the vicinity of surface of the front side or back side.

Therefore, the applicants of the present application applied the sameheat treatment as the above heat treatment as the heat treatment forobtaining an interstitial oxygen concentration of 8 ppma or less. As aresult, it was found that a high resistivity DZ-IG wafer having a highresistivity of 100 Ω·cm or more and having a DZ layer free from crystaldefects near the surface and an oxide precipitate layer in which oxideprecipitates are sufficiently precipitated could be obtained.

It was considered that such a high resistivity DZ-IG wafer couldsufficiently serve as an alternative of SOI wafers for mobilecommunications. However, subsequent investigations revealed that, ifsuch a DZ-IG wafer was subjected to a heat treatment during the deviceproduction process, the resistivity near the wafer surface was extremelyreduced as the case may be, and thus sufficient high resistivity may notbe obtained.

Further, it was also found that, although the DZ layer formed by such aheat treatment was surely made defect free as for defects originatedfrom oxide precipitates, grown-in defects called COP (Crystal OriginatedParticle) were not eliminated and still remained.

COP is a void of 0.1 μm order size formed by aggregation of excessivevacancies during the growth of CZ silicon single crystals, and theinternal surface thereof is covered with a thin oxide film. Further, itis known that, if a device is formed on a portion where such grown-indefects exist, device characteristics such as oxide dielectric breakdownvoltage are degraded.

DISCLOSURE OF THE INVENTION

The present invention was accomplished in order to solve these problems,and its object is to provide a method for producing a silicon wafer inwhich a DZ layer of high quality made defect free not only for oxideprecipitates but also for COPs is formed in the vicinity of the wafersurface, oxide precipitates are formed in the bulk portion at asufficient density and thereby high gettering ability can be obtained,and which can maintain high resistivity even after the wafer issubjected to a heat treatment for device production, and thereby providea high resistivity DZ-IG wafer of high quality at a thus-far unknownlevel, which can serve as an alternative of SOI wafer for mobilecommunications.

In order to achieve the aforementioned object, the present inventionprovides a method for producing a silicon wafer, which comprises growinga silicon single crystal ingot having a resistivity of 100 Ω·cm or moreand an initial interstitial oxygen concentration of 10 to 25 ppma anddoped with nitrogen by the Czochralski method, processing the siliconsingle crystal ingot into a wafer, and subjecting the wafer to a heattreatment so that a residual interstitial oxygen concentration in thewafer should become 8 ppma or less.

If nitrogen is doped in a silicon single crystal as described above,sizes of grown-in defects (COPs) become small, and it becomes easy toeliminate them by a heat treatment. In addition, formation and growth ofoxygen precipitation nuclei can be attained to a certain degree duringthe crystal growth. Thus, it becomes possible to form a DZ layer of highquality by a heat treatment at a temperature lower than that of theconventional DZ-IG treatment by a three-step heat treatment (formationof DZ layer (high temperature)+formation of precipitation nuclei (lowtemperature)+growth of precipitates (moderate temperature)), and it alsobecomes possible to grow oxide precipitates of a sufficient density inthe bulk portion by a heat treatment for a short period of time.Therefore, the transition region between the DZ layer and the oxideprecipitate region can be made to have a narrow and sharp profile, andthe amount of interstitial oxygen in the whole transition region can bemade small. Thus, the influence of oxygen acting as donor can bereduced. Furthermore, it also becomes possible to enlarge the acceptablerange of the initial interstitial oxygen concentration that can providean interstitial oxygen concentration of 8 ppma (JEIDA: Japan ElectronicIndustry Development Association Standard) or less for a specific heattreatment.

The present invention also provides a method for producing a siliconwafer, which comprises growing a silicon single crystal ingot having aresistivity of 100 Ω·cm or more and an initial interstitial oxygenconcentration of 8 ppma or less and doped with nitrogen by theCzochralski method, processing the silicon single crystal ingot into awafer, and subjecting the wafer to a heat treatment to form an oxideprecipitate layer in a bulk portion of the wafer.

If nitrogen is doped in a silicon single crystal as described above, theoxygen precipitation is more promoted when the wafer is subjected to anoxygen precipitation heat treatment compared with a wafer not doped withnitrogen, even if the wafer is a wafer of low oxygen content having aninitial interstitial oxygen concentration of 8 ppma or less, and thus itbecomes possible to form oxide precipitates at a sufficient density.Moreover, since the wafer contains oxygen at a low concentration, theoxide films on the internal surfaces of COPs formed during the crystalgrowth become thin, and it becomes easy to eliminate COPs by a heattreatment. For these reasons, it becomes possible to form a DZ layer ofhigh quality and oxide precipitates at a sufficient density in the bulkportion by a heat treatment at a relatively lower temperature for ashorter period of time compared with those used in the conventionaltechniques. Furthermore, since the interstitial oxygen concentration isoriginally 8 ppma or less, substantially no fluctuation of resistivitydue to oxygen acting as donor is caused in the device productionprocess. Moreover, the method also has an advantage that the problem oflow resistance of the wafer to slip dislocations generated by a heattreatment due to the low oxygen concentration in a wafer not doped withnitrogen can also be covered by the use of the milder heat treatmentconditions (low temperature and short time).

In the aforementioned methods, nitrogen is preferably doped at aconcentration of 1×10¹² to 5×10¹⁵ number/cm³.

This is because, if the nitrogen concentration is less than 1×10¹²number/cm³, the effect is not so remarkable compared with a case notusing the nitrogen doping, and if it exceeds 5×10¹⁵ number/cm³, thesingle crystallization during the pulling of the crystal may beinhibited, or it may make continuous operation unstable. Morepreferably, the nitrogen concentration should be less than 1×10¹⁴number/cm³. This is because, if the nitrogen concentration is 1×10¹⁴number/cm³ or more, the amount of oxygen-nitrogen donors formed by aheat treatment at around 600° C. increases, and they may reduce theresistivity. That is, it is known that about 10% of the doped nitrogencontributes to the formation of oxygen-nitrogen donors, and a dopingamount of 1×10¹⁴ number/cm³ may form 1×10¹³ number/cm³ ofoxygen-nitrogen donors. If all of these donors are activated, there iscaused fluctuation of the resistivity in the order of several hundredsΩ·cm. However, it can be conversely said that, if the amount of thegenerated donors is the above level or less, they show substantially noinfluence.

Further, in the aforementioned method, the heat treatment is preferablyperformed at a temperature of 1000 to 1200° C. for 1 to 20 hours inhydrogen gas, argon gas or a mixed gas atmosphere of hydrogen gas andargon gas.

If the heat treatment is performed in hydrogen gas, argon gas or a mixedgas atmosphere thereof as described above, grown-in defects at the wafersurface and in the vicinity of the wafer surface can be effectivelyeliminated, and the oxide precipitates in the bulk portion can be grownat the same time. In this case, if the heat treatment temperature islower than 1000° C., a heat treatment for a long period of timeexceeding 20 hours is required in order to sufficiently eliminate thegrown-in defects. Further, although the grown-in defects can besufficiently eliminated by a heat treatment for about 1 hour at a heattreatment temperature of 1200° C., if the temperature exceeds 1200° C.,the oxygen precipitation nuclei formed by the effect of the nitrogendoping during the crystal growth become likely to melt again, and thusit becomes difficult to obtain a sufficient oxide precipitate densityafter the heat treatment. Therefore, the heat treatment temperature ispreferably 1000 to 1200° C.

A silicon wafer produced by the production method of the presentinvention described above has a high resistivity of 100 Ω·cm or more anda low interstitial oxygen concentration of 8 ppma or less. Therefore, itcan be a high resistivity DZ-IG wafer of high quality in whichresistivity is not reduced by oxygen acting as donor during the deviceproduction process, and which contains substantially no grown-in defectin the DZ layer near the wafer surface.

As explained above, according to the present invention, there can beobtained a CZ silicon wafer in which fluctuation of resistivity due tointerstitial oxygen acting as donor is suppressed even after the waferis subjected to a heat treatment for device production. This effect isextremely effective for a high resistivity CZ wafer having a resistivityof 100 Ω·cm or more, and it enables use of the wafer as an alternativeof SOI wafer for mobile communications.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a graph representing experimental results showing therelationship between the initial interstitial oxygen concentration andthe residual interstitial oxygen concentration of a CZ wafer subjectedto a usual oxygen precipitation heat treatment.

FIG. 2 is a graph representing experimental results showing therelationship between the initial interstitial oxygen concentration andthe density of precipitates of a CZ wafer subjected to a usual oxygenprecipitation heat treatment.

FIG. 3 is a graph showing the relationship between the depth fromsurface and the resistivities before and after heat treatment in aconventional silicon wafer.

FIG. 4 is a graph showing the relationship between the depth fromsurface and the absolute value of oxygen concentration in a conventionalsilicon wafer.

FIG. 5 is a schematic view showing precipitate distribution along thedepth direction in a conventional silicon wafer.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereafter, the present invention will be explained in detail.

As described above, in order to obtain a “high resistivity DZ-IG wafer”that can realize, as a bulk wafer, performance at a level equivalent tothat of an SOI wafer for mobile communications utilizing a highresistivity wafer as the base wafer, the inventors of the presentinvention applied a three-step heat treatment usually performed as aheat treatment for obtaining an interstitial oxygen concentration of 8ppma or less to a CZ silicon wafer of high resistivity as a trial. Theheat treatment of the third step for growing oxide precipitates wasperformed by divided two stages at temperatures of 800° C. and 1000° C.

As a result, in the wafer immediately after the three-step heattreatment, a DZ layer was formed in the vicinity of the wafer surfacewhile the high resistivity was maintained, and an IG layer (oxideprecipitate region) was formed in the bulk portion. Thus, a desired highresistivity DZ-IG wafer was obtained (FIG. 3( a)). However, when a heattreatment simulating a device production heat treatment was applied tothis wafer, it was found that the resistivity might extremely decreasedin the vicinity of the wafer surface as the case may be (FIG. 3( b)).

It was expected that the resistivity decrease was caused becauseinterstitial oxygen existing in the wafer became donor. Therefore, theinventors of the present invention measured and examined distribution ofabsolute value of oxygen concentration along the depth direction in awafer after the three-step heat treatment, in which the resistivitydecreased, by using a secondary ion mass spectroscopy (SIMS) apparatus(FIG. 4). Further, the wafer was subjected to angle polishing andpreferential etching, and then distribution of oxide precipitates (etchpits) along the depth direction was observed. The results areschematically shown in FIG. 5.

From the results shown in FIGS. 4 and 5, it can be seen that a region ofabout 20 μm from the surface is the DZ layer, a deeper region of a depthof about 30 μm or more from the surface is the oxide precipitate layer,and a region between them of a depth of about 20 to 30 μm from thesurface is the transition region (a region that does not fully become aDZ layer, in which a few oxide precipitates exist) in the wafer referredto in FIG. 4. A region around the transition region corresponded to aregion in which resistivity was extremely decreased after the heattreatment simulating a device production heat treatment. When theinterstitial oxygen concentration in this region was measured later byinfrared absorption spectroscopy, it was found to be a portion where theinterstitial oxygen concentration exceeded 8 ppma (4×10¹⁷ atoms/cm³).

Furthermore, when COPs in a region of a depth of several micrometersfrom the surface were measured by using a particle counter before andafter the three-step heat treatment, there is almost no change wasobserved, and thus it was confirmed that COPs remained in the wafersurface portion.

That is, it is considered that, even if the interstitial oxygen in theDZ layer near the surface is out-diffused, and the interstitial oxygenconcentration in the bulk portion becomes sufficiently low due toprecipitation of oxygen as oxide precipitates, the interstitial oxygenconcentration is still high in the transition region between them evenafter the three-step heat treatment, and therefore the resistivity isdecreased by oxygen becoming donor.

Therefore, it was considered that, for the purpose of surely obtaining ahigh resistivity DZ-IG wafer, the interstitial oxygen concentration ofnot only the DZ layer and the oxide precipitate layer but also thetransition region between the both should be made to be 8 ppma or less,or if the aforementioned transition region could be made to have a widthas narrow as possible and a profile as sharp as possible, the amount ofinterstitial oxygen as the whole transition region would become small,and thus the influence of oxygen that became donor could also be madesmall. In addition, it was also considered that it was necessary todecrease COPs in the DZ layer.

Therefore, the inventors of the present invention investigatedproduction conditions for a silicon wafer that satisfy theserequirements. As a result, they conceived doping of a silicon singlecrystal with nitrogen during the growth of the crystal by the CZ method.That is, it has been pointed out that, if nitrogen is doped in a siliconsingle crystal, the aggregation of oxygen atoms in the silicon ispromoted, and thus the oxide precipitate density increases (T. Abe andH. Takeno, Mat. Res. Soc. Symp. Proc. Vol. 262, 3, 1992). It is thoughtthat this effect is obtained because the aggregation process of oxygenatoms is shifted from that consisting of homogenous nucleus formation tothat consisting of heterogenous nucleus formation utilizing nitrogenimpurities as nuclei. Furthermore, it is also known that, if nitrogen isdoped in a single crystal, sizes of crystal defects such as COPs becomesmall.

Therefore, if a silicon single crystal is doped with nitrogen during thegrowth thereof, formation and growth of oxygen precipitation nuclei canbe attained to a certain extent during the crystal growth, thus it isexpected that the precipitation of interstitial oxygen is promoted alsoin the transition region between the DZ layer and the oxide precipitatelayer to sufficiently reduce the interstitial oxygen concentration, andit is considered that the width of the transition region can be narrowedso that the region should have a sharp profile. Further, it isconsidered that, since sizes of grown-in defects such as COPs are madesmall by the nitrogen doping, it becomes easier to eliminate them by asubsequent heat treatment. Therefore, the inventors of the presentinvention performed the following experiments concerning nitrogen-dopedCZ wafers.

EXPERIMENTAL EXAMPLE 1

FIG. 1 shows experimental results representing relationship between theinitial interstitial oxygen concentration and the residual interstitialoxygen concentration in various CZ wafers having an initial interstitialoxygen concentration of 8 to 21 ppma after they were subjected to usualheat treatments for oxygen precipitation, i.e., a heat treatment at 780°C. for 3 hours and a heat treatment at 1000° C. for 16 hours under anitrogen atmosphere (containing 3% of oxygen). Two kinds of the nitrogenconcentrations, i.e., 1×10¹³ to 9×10¹³ number/cm³ and no nitrogendoping, were used.

From the results shown in FIG. 1, it can be seen that if nitrogen is notdoped, the residual interstitial oxygen concentration cannot be made 8ppma or less after the aforementioned heat treatments unless the initialinterstitial oxygen concentration is 19 ppma or more, whereas ifnitrogen is doped, the range of acceptable initial interstitial oxygenconcentration is enlarged to the range of 15 ppma or more. Further, itwas confirmed that, by using a longer heat treatment time, the range ofacceptable initial interstitial oxygen concentration could be enlargedto the range of 10 ppma or more.

EXPERIMENTAL EXAMPLE 2

FIG. 2 shows experimental results representing relationship between theinitial interstitial oxygen concentration and the density ofprecipitates in various CZ wafers having an initial interstitial oxygenconcentration of 4 to 19 ppma after they are subjected to usual heattreatments for oxygen precipitation, i.e., a heat treatment at 780° C.for 3 hours and a heat treatment at 1000° C. for 16 hours under anitrogen atmosphere (containing 3% of oxygen), angle polishing andpreferential etching, in which the measured density of precipitates inthe oxygen precipitation region was converted into volume density. Twokinds of the nitrogen concentrations, i.e., 1×10¹³ to 9×10¹³ number/cm³and no nitrogen doping, were used.

From the results shown in FIG. 2, it can be seen that the oxideprecipitate density is markedly increased by the nitrogen doping, evenif the initial interstitial oxygen concentration is the same. It can beseen that, in particular, even with a low initial interstitial oxygenconcentration of 8 ppma or less, a precipitate density of 1×10⁸number/cm³ can be obtained by the nitrogen doping.

Based on the results shown in FIG. 1 mentioned above, it is consideredthat it is expected that oxide precipitates are not substantially formedin a non-nitrogen-doped wafer of a low oxygen concentration having aninitial interstitial oxygen concentration of 8 ppma or less, but whennitrogen was doped, oxygen precipitation nuclei (oxide precipitates)stable at a high temperature had been already formed in the as-grownstate, and therefore oxide precipitates having sizes detectable as oxideprecipitates were obtained even by an extremely small degree of nucleusgrowth in such an extent that the growth could not be detected as anamount of precipitated oxygen after the subsequent heat treatment.

From these results, it is expected that, as for wafers obtained from asingle crystal having an initial interstitial oxygen concentration ofabout 10 to 25 ppma, if the crystal is grown with nitrogen doping, theoxygen precipitation is promoted during the subsequent heat treatmentfor oxygen precipitation, thus the residual interstitial oxygenconcentration can be reduced also in the transition region, and asufficiently narrow and sharp profile of the transition region can beobtained. Further, it can be seen that, as also for wafers obtained froma single crystal having an initial interstitial oxygen concentration of8 ppma or less, if nitrogen is doped, sufficient oxygen precipitationcan be obtained by the subsequent heat treatment for oxygen, andsufficient gettering effect can be secured.

Further, although the quality of the DZ layer, especially the presenceor absence of COP, was not evaluated in the experiments described above,it is known that sizes of COPs are made smaller by nitrogen doping, andthus it becomes easier to eliminate them by a heat treatment. Therefore,the inventors of the present invention considered that, if eliminationof COPs and formation of oxide precipitates were performedsimultaneously under conditions under which COPs were easily eliminated,and oxide precipitates are sufficiently formed, i.e., by using atemperature not so high as the DZ formation heat treatment of theconventional three-step heat treatment (high temperature heat treatment)instead of the heat treatments used in the aforementioned experiments,the transition region could be made to have a narrow width and a sharpprofile, as a result, high resistivity could be maintained, and thedesired CZ silicon wafer could eventually be obtained, and theyaccomplished the present invention.

The present inventions will be further explained hereafter. However, thepresent invention is not limited by these explanations.

First, a silicon single crystal ingot is pulled by the known CZ methodor the known MCZ method where a single crystal is pulled while amagnetic field is applied to a melt in the CZ method to controlconvection of the silicon melt, so that the silicon single crystal ingotshould have a desired high resistivity of 100 Ω·cm or more and aninitial interstitial oxygen concentration of 10 to 25 ppma. Thesepulling methods are methods comprising bringing a seed crystal intocontact with a melt of polycrystalline silicon raw material contained ina quartz crucible and slowly pulling the seed crystal with rotation toallow growth of a single crystal ingot of a desired diameter. A desiredinitial interstitial oxygen concentration can be obtained by usingconventional techniques. For example, a crystal having a desired oxygenconcentration can be obtained by suitably adjusting parameters such asrotational speed of the crucible, flow rate of introduced gas,atmospheric pressure, temperature distribution and convection of siliconmelt and strength of the magnetic field to be applied.

In order to pull a silicon single crystal having an initial interstitialoxygen concentration of 8 ppma or less (also referred to as “low oxygenconcentration” hereafter), the parameters to be controlled during thecrystal growth are similar to those for the case where the interstitialoxygen concentration is 10 to 25 ppma (also referred to as “high oxygenconcentration” hereafter) is pulled as mentioned above. However, inorder to stably pull such a crystal of low oxygen concentration, the MCZmethod is usually used.

The simultaneous doping with nitrogen as well as oxygen can be easilyperformed by preliminarily adding nitride such as wafers having nitridefilms into the raw material polycrystal contained in a quartz crucible.The concentration of nitrogen to be doped in a pulled crystal can becalculated from the amount of nitride introduced into the raw materialpolycrystal or crucible, the segregation coefficient of nitrogen and soforth.

The CZ silicon single crystal ingot obtained as described above issliced by using a cutting machine such as a wire saw or inner diameterslicer, and subjected to steps of chamfering, lapping, etching,polishing and so forth to be processed into CZ silicon single crystalwafers according to conventional techniques. Of course, these steps aremere examples, and there may be used various other steps such ascleaning step and heat treatment step. Further, the steps are used withsuitable modification including the alteration of the order of steps,omission of some steps and so forth according to the purpose.

Then, to a wafer of high oxygen concentration, a heat treatment thatprovides a residual interstitial oxygen concentration of 8 ppma or lessis applied. The heat treatment that provides a residual interstitialoxygen concentration of 8 ppma or less used in this case cannot benecessarily specified, because the residual interstitial oxygenconcentration varies depending on the initial interstitial oxygenconcentration and thermal history during crystal growth of a wafer to besubjected to a heat treatment. However, it can be determined byexperiments according to the initial interstitial oxygen concentration,thermal history and so forth.

In addition, in the present invention, not only the formation of oxideprecipitates by a heat treatment, but also elimination of COPs by a heattreatment must be considered. The heat treatment for eliminating COPs ispreferably a heat treatment at a high temperature in hydrogen gas, argongas or a mixed gas thereof. However, at an unduly high temperature,there arise problems that oxide precipitates become unlikely to beformed, and the width of the transition region is broadened. Therefore,the heat treatment temperature is preferably 1000 to 1200° C. Even withsuch a relatively low temperature, COPs can be sufficiently eliminated,because the sizes of COPs are made small by the effect of the nitrogendoping, and interstitial oxygen can also be reduced by out-diffusionthereof. Further, if hydrogen gas, argon gas or a mixed gas thereof isused, the out-diffusion profile of oxygen abruptly changes at the wafersurface, and thus a transition region having a sharper profile can beformed.

The aforementioned heat treatment can also be dividedly performed, forexample, with two stages at 1200° C. and 1000° C., to sufficientlyeliminate COPs at a high temperature of the first stage, and thensufficiently grow oxide precipitates at a low temperature of the secondstage.

On the other hand, as for a wafer of low oxygen concentration, change ofresistivity depending on the width of the transition region need not beconsidered, COPs can be easily eliminated by a heat treatment at atemperature as high as possible within a range that provides sufficientformation of oxide precipitates. However, in order to obtain sufficientoxygen precipitates, the temperature is preferably 1000 to 1200° C.

These heat treatments can be performed by using a usual vertical typefurnace or a usual horizontal type furnace (diffusion furnace), whichenables a simultaneous heat treatment of many wafers.

The present invention will be specifically explained hereafter withreference to the following examples of the present invention andcomparative examples. However, the present invention is not limited bythese.

EXAMPLE 1

Silicon wafers having nitride films were introduced into raw materialpolycrystal, and a nitrogen-doped silicon single crystal was pulled bythe CZ method (without applying magnetic field). The silicon singlecrystal was processed by a usual method to produce a CZ silicon waferhaving a diameter of 200 mm, an initial interstitial concentration of 18ppma (JEIDA), a nitrogen concentration of 8×10¹³ number/cm³ (calculatedvalue) and a resistivity of 2500 Ω·cm.

This wafer was subjected to a heat treatment at 1100° C. for 2 hours anda heat treatment at 1000° C. for 16 hours under a 100% argon atmosphereand further subjected to heat treatments simulating a device productionprocess (heat treatments at 1200° C. for 1 hour and at 450° C. for 5hours) under a nitrogen atmosphere (mixed with 3% of oxygen) in avertical type heat treatment furnace. For the silicon wafer obtained asdescribed above, the residual interstitial oxygen concentration wasmeasured by infrared absorption spectroscopy for the whole wafer, and itwas confirmed that the residual interstitial oxygen concentration in thewafer was 8 ppma or less.

Thereafter, the wafer after the heat treatments was subjected to anglelapping, and then resistivity was measured by the spreading resistancemeasurement method for a portion from the surface to a depth of 100 μm.As a result, it was confirmed that every region in the wafer had aresistivity of 2000 Ω·cm or more. Subsequently, the surface undergoneangle polishing was subjected to preferential etching and observed byusing a optical microscope. As a result, the transition region had anarrow width of about 5 μm, i.e., a sharp profile, and the defectdensity in the oxygen precipitation region was a sufficient value, i.e.,5 to 8×10⁹ number/cm³. That is, it can be seen that the silicon wafer ofExample 1 was a DZ-IG wafer scarcely influenced by the resistivityreduction due to the oxygen donor and having sufficient getteringability.

Further, the surface of the wafer after the heat treatments was polishedby about 3 μm, and density of COPs existing on the polished surface andhaving a size of 0.12 μm or more as the diameter were measured by usinga particle counter (SP1, produced by KLA Tencor Corporation). Themeasured COP density was an extremely low density, i.e., 0.06number/cm².

COMPARATIVE EXAMPLE 1

A silicon single crystal was pulled with the same conditions as those ofExample 1 except that nitrogen was not doped, and processed by a usualmethod to produce a CZ silicon wafer having a diameter of 200 mm, aninitial interstitial concentration of 18 ppma (JEIDA) and a resistivityof 2500 Ω·cm.

This wafer was subjected to a three-step heat treatment (DZ-IGtreatment) comprising a heat treatment at 1150° C. for 4 hours, a heattreatment at 650° C. for 6 hours and a heat treatment at 1000° C. for 16hours under a nitrogen atmosphere (mixed with 3% of oxygen) and furthersubjected to heat treatments simulating a device production process(heat treatments at 1200° C. for 1 hour and at 450° C. for 5 hours) in avertical type heat treatment furnace. For the silicon wafer obtained asdescribed above, the residual interstitial oxygen concentration wasmeasured by infrared absorption spectroscopy for the whole wafer, and itwas confirmed that the residual interstitial oxygen concentration in thewafer was 8 ppma or less.

Thereafter, the wafer after the heat treatments was subjected to anglelapping, and then resistivity was measured by the spreading resistancemeasurement method for a portion from the surface to a depth of 100 μm.As a result, it was confirmed that the resistivity was decreased to tenand several Ω·cm in a region of a depth of 20 to 40 μm from the surface.Subsequently, the surface undergone angle polishing was subjected topreferential etching and observed by using a opical microscope. As aresult, the region in which resistivity was decreased substantiallycorresponded to the transition region. The interstitial oxygenconcentration was confirmed again by infrared absorption spectroscopyonly for the transition region, it was found that the region is aportion where the interstitial oxygen concentration exceeded 8 ppma(4×10¹⁷ atom/cm³). Based on this, it is considered that the amount ofinterstitial oxygen becoming donor was large in that portion, thus theconductivity type was reversed from p-type into n-type, and thereforethe resistivity was further decreased. The measurement of theinterstitial oxygen concentration in the transition region by infraredabsorption spectroscopy can be performed by, for example, a measurementmethod of using a bonded wafer obtained by eliminating the DZ layer bypolishing, bonding the obtained surface to an FZ wafer and eliminatingthe oxide precipitate layer.

Further, the surface of the wafer after the heat treatments was polishedby about 3 μm, and density of COPs existing on the polished surface andhaving a size of 0.12 μm or more as the diameter were measured by usinga particle counter (SP1, produced by KLA Tencor Corporation). Themeasured COP density was 4.3 number/cm², which was substantially thesame density as the density before the heat treatments.

EXAMPLE 2

Silicon wafers having nitride films were introduced raw materialpolycrystal, and a nitrogen-doped silicon single crystal was pulled bythe MCZ method. The silicon single crystal was processed by a usualmethod to produce a CZ silicon wafer having a diameter of 200 mm, aninitial interstitial concentration of 6 ppma (JEIDA), a nitrogenconcentration of 9×10¹³ number/cm³ (calculated value) and a resistivityof 1500 Ω·cm.

This wafer was subjected to a heat treatment at 800° C. for 4 hoursunder a nitrogen atmosphere and a heat treatment at 1100° C. for 16hours under an argon atmosphere (mixed with 3% of hydrogen) and furthersubjected to heat treatments simulating a device production process(heat treatments at 1200° C. for 1 hour and at 450° C. for 5 hours) in avertical type heat treatment furnace. For the silicon wafer obtained asdescribed above, the residual interstitial oxygen concentration wasmeasured by infrared absorption spectroscopy for the whole wafer, and itwas confirmed that the residual interstitial oxygen concentration in thewafer was 8 ppma or less.

Thereafter, the wafer after the heat treatments was subjected to anglelapping, and resistivity was measured by the spreading resistancemeasurement method for a portion from the surface to a depth of 100 μm.As a result, it was confirmed that every region in the wafer had aresistivity of 1000 Ω·cm or more. Subsequently, the surface undergoneangle polishing was subjected to preferential etching and observed byusing a optical microscope. As a result, the defect density in the oxideprecipitate region was a sufficient value, i.e., 1 to 5×10⁸ number/cm³.That is, it can be seen that the silicon wafer of Example 2 was also aDZ-IG wafer scarcely influenced by the resistivity reduction due to theoxygen donor and having sufficient gettering ability, even though thesilicon single crystal was grown with a low oxygen concentration.

Further, the surface of the wafer after the heat treatments was polishedby about 3 μm, and density of COPs existing on the polished surface andhaving a size of 0.12 μm or more as the diameter were measured by usinga particle counter (SP1, produced by KLA Tencor Corporation). Themeasured COP density was extremely low density, i.e., 0.1 number/cm².

COMPARATIVE EXAMPLE 2

A silicon single crystal was pulled with the same conditions as those ofExample 2 except that nitrogen was not doped, and processed by a usualmethod to produce a CZ silicon wafer having a diameter of 200 mm, aninitial interstitial concentration of 6 ppma (JEIDA) and a resistivityof 1500 Ω·cm.

This wafer was subjected to a three-step heat treatment (DZ-IGtreatment) comprising a heat treatment at 1150° C. for 4 hours, a heattreatment at 650° C. for 6 hours and a heat treatment at 1000° C. for 16hours under a nitrogen atmosphere (mixed with 3% of oxygen) and furthersubjected to heat treatments simulating a device production process(heat treatments at 1200° C. for 1 hour and at 450° C. for 5 hours) in avertical type heat treatment furnace. For the silicon wafer obtained asdescribed above, the residual interstitial oxygen concentration wasmeasured by infrared absorption spectroscopy for the whole wafer, and itwas confirmed that the residual interstitial oxygen concentration in thewafer was 8 ppma or less.

Thereafter, the wafer after the heat treatments was subjected to anglelapping, and resistivity was measured by the spreading resistancemeasurement method for a portion from the surface to a depth of 100 μm.As a result, it was confirmed that every region in the wafer had aresistivity of 1000 Ω·cm or more. However, when the surface undergoneangle polishing was subjected to preferential etching and observed byusing a optical microscope, the defect density in the oxide precipitateregion was found to be extremely low, i.e., 1 to 5×10⁶ number/cm³. Thatis, it can be seen that, although the silicon wafer of ComparativeExample 2 was a wafer not substantially influenced by the resistivityreduction due to the oxygen donor, it was a wafer that did not havesufficient gettering ability, since the silicon single crystal was grownwith a low oxygen concentration, and thus oxygen precipitation wasunlikely to occur in the bulk portion.

Further, the surface of the wafer after the heat treatments was polishedby about 3 μm, and density of COPs existing on the polished surface andhaving a size of 0.12 μm or more as the diameter were measured by usinga particle counter (SP1, produced by KLA Tencor Corporation). Themeasured COP density was 4.0 number/cm², which was substantially thesame density as the density before the heat treatments.

The present invention is not limited to the embodiments described above.The above-described embodiments are mere examples, and those having thesubstantially same configuration as that described in the appendedclaims and providing the similar functions and advantages are includedin the scope of the present invention.

1. A method for producing a silicon wafer, which comprises growing asilicon single crystal ingot having a resistivity of 100 Ω·cm or moreand an initial interstitial oxygen concentration of 10 to 25 ppma anddoped with nitrogen by the Czochralski method, processing the siliconsingle crystal ingot into a wafer, and subjecting the wafer to a heattreatment so that a residual interstitial oxygen concentration in thewafer becomes 8 ppma or less.
 2. A method for producing a silicon wafer,which comprises growing a silicon single crystal ingot having aresistivity of 100 Ω·cm or more and an initial interstitial oxygenconcentration of 8 ppma or less and doped with nitrogen by theCzochralski method, processing the silicon single crystal ingot into awafer, and subjecting the wafer to a heat treatment to form an oxideprecipitate layer in a bulk portion of the wafer.
 3. The methodaccording to claim 1, wherein nitrogen is doped at a concentration of1×10¹² to 5×10¹⁵ number/cm³.
 4. The method according to claim 2, whereinnitrogen is doped at a concentration of 1×10¹² to 5×10¹⁵ number/cm³. 5.The method according to claim 1, wherein the heat treatment is performedat a temperature of 1000 to 1200° C. for 1 to 20 hours in hydrogen gas,argon gas or a mixed gas atmosphere of hydrogen gas and argon gas. 6.The method according to claim 2, wherein the heat treatment is performedat a temperature of 1000 to 1200° C. for 1 to 20 hours in hydrogen gas,argon gas or a mixed gas atmosphere of hydrogen gas and argon gas. 7.The method according to claim 3, wherein the heat treatment is performedat a temperature of 1000 to 1200° C. for 1 to 20 hours in hydrogen gas,argon gas or a mixed gas atmosphere of hydrogen gas and argon gas. 8.The method according to claim 4, wherein the heat treatment is performedat a temperature of 1000 to 1200° C. for 1 to 20 hours in hydrogen gas,argon gas or a mixed gas atmosphere of hydrogen gas and argon gas.
 9. Asilicon wafer produced by the production method according to claim 1.10. A silicon wafer produced by the production method according to claim2.
 11. A silicon wafer produced by the production method according toclaim
 3. 12. A silicon wafer produced by the production method accordingto claim
 4. 13. A silicon wafer produced by the production methodaccording to claim
 5. 14. A silicon wafer produced by the productionmethod according to claim
 6. 15. A silicon wafer produced by theproduction method according to claim
 7. 16. A silicon wafer produced bythe production method according to claim 8.